The invention relates to a method for determining an amplified spontaneous emission (ASE) in an optical fiber amplifier, an associated data medium, and a single-stage and a multi-stage optical fiber amplifier.
Modern Erbium-doped fiber amplifiers for WDM systems with very large ranges generally have three amplifier stages with amplification fibers, which are separated by components such as variable attenuators and dispersion-compensating fibers, and have a combined gain and output power regulator.
For the purpose of measuring total signal power at the amplifier's input and the output, photodiodes are generally used, although their measurement signals can be corrupted because the signals are overlaid with noise in the form of amplified spontaneous emission, ASE. The relative error produced by this can be large, above all when a WDM (WDM=Wavelength Division Multiplex) signal which is to be amplified has a small number of channels, in particular because modern receivers with improved error-correction methods (such as using EFEC=Enhanced Forward Error Correction) permit low signal-to-noise ratios.
In order to be able to set the required signal output power, it is therefore necessary to correct the measurement signal for the overlaid amplified spontaneous emission, ASE, by estimating as accurately as possible the amplified spontaneous emission ASE generated in the individual amplifiers.
The known methods use the following approximation for calculating the ASE power (power of the amplified spontaneous emission, ASE) produced in an EDFA (C-band amplifier with an optical bandwidth Bopt=4.44 THz):PASE=ν·Bopt·nf·g=0.57045·10−3·nf·g{MW}where Bout stands for the optical bandwidth, ν is the mean carrier frequency of the signals (typically: 193.40 THz, 1550 nm), the variable g represents the EDFA gain and nf is the EDFA noise figure.
In general, the EDFA noise figure nf is available in tabular form for a few operating points, in terms of the EDFA input power and the EDFA gain, for 40-channel mode. An exact calculation of the ASE power generated is therefore not possible, because:                variation in the passive losses in the EDFA can produce significant changes,        spectral shaping factors (e.g. for the gain-flattening filter used) are left out of consideration,        for operation with a low number of channels (1 . . . 10) the EDFA noise figure can deviate significantly from the tabulated values for 40-channel operation, i.e. the noise figure depends not only on the input power and on the gain but also on the actual channel usage.        
In the case of a three-stage optical fiber amplifier, any characterization of the amplifier simply as a function of the overall gain from the input to the output and as a function of the input power relatively quickly runs up against accuracy limits, because for a given gain the passive losses between the individual stages can vary greatly, which has a strong influence on the ASE power produced.